Mixing impellers are in wide use in industry. Examples of industrial mixing impellers include designs which have a central hub and two, three, four or more radially extending blade type structures. These blades may be flat, angled, and in some cases have a wing or propeller shape. Typically, the impellers extend radially outwardly from a motor-driven shaft and are submerged inside a material to be mixed. Oftentimes the impellers are in an at least partially liquid mix which is being confined in a vessel, which may be holding the material in a batch process or a continuous process.
Some mixing vessels are closed, and the impeller is moved into and out of the vessel through a small opening by folded the blades of the impeller around the drive shaft. Impellers having folding blades are known for being mounted at the end of the drive shaft. However, known designs for impellers having such folding blades tend to be for small impellers, and such known designs for small folding blade impellers tend to be inefficient and unsophisticated, and require a large opening to pass into and out of the closed vessel.